Restricted repertoires of antigen specific receptors on B and T cells have been proposed to define discrete lymphocyte subpopulations at the frontier between innate and adaptive immunity. Notable examples of this are NK-T cells that use an invariant (TCR chain with a CDR3 of constant length and limited Vbeta segments. The activation and in vivo development of NK-T cells in mice is dependent upon the class Ib molecule, CDld. A remarkable story has recently emerged of a new alpha/beta T population termed mucosal-associated invariant T cells or MAlT cells. Interestingly, MAlT cells are predominantly expressed in the gut and are missing in germ-free mice. Furthermore, MAlT cells, like NK-T cells, have an invariant (TCR rearrangement of constant CDR3 length and limited Vbeta usage. And finally, activation and in vivo development of MAlT cells is dependent upon the newly characterized class Ib molecule, MR1. MR1 is highly conserved among mammals, and non-MHC encoded. We recently reported that MR1 has limited cell surface expression in transfected cell lines, is associated with the peptide-loading complex, and undergoes a ligand induced folding event similar to class la molecules. Furthermore, recombinant MR1 was obtained, but only in insect cells grown in highly supplemented media. These biochemical properties of MR1and the aforementioned analogies with CD1, raise the interesting possibility that MR1 presents a specific ligand from commensal flora to MAlT cells and thereby regulates mucosal immune responses. The proposed studies in this grant will define i) where endogenous MR1 is expressed, ii)the effector function of MAlT cells, iii) the nature of the putative MR1 ligand and whether it is bacteria-specific, and iv) how the MHC fold of MR1 functionally interacts with a putative ligand and the alpha/beta TCR of MAlT cells.